The present invention relates to electromagnetic launchers, and more specifically to electromagnetic railguns, designed to accelerate solid bodies to higher velocities than those of conventional guns.
Electromagnetic launchers have been widely investigated because of the potential to achieve velocities exceeding those that can be practically attained in powder and other thermodynamic guns, in which the maximum velocity is limited by the specific energy of the known propellants. If high enough velocities could be obtained efficiently, electromagnetic launchers would have important military, scientific and commercial applications. The applications include testing and developing new materials, earth-to-orbit launching of materials, such as fuel, building and hazardous materials, and simulation of nuclear fusion impact with velocities on the order of 100 km/s.
The best known and most investigated electromagnetic launchers belong to the railgun family, in which projectiles are accelerated by an electromagnetic driving force applied to plasma or metallic armatures. The armature closes a circuit formed by a pulsed electric power source and a pair of parallel, elongated conducting rails. The power source generates current pulses in the rail-armature circuit. The rails form a railgun bore along which the projectile slides as it is accelerated by electromagnetic forces. U.S. Pat. No. 4,953,441 provides a brief description of the current state of railgun technology and a discussion of potential advantages of railguns with solid armatures.
Early railguns used metal armatures. During acceleration, the metal armatures were heated by the current pulse. If the contact is ideal and the current in the metal armature is distributed uniformly, there exists a theoretical velocity limit due to thermal degradation of the armature material. The theoretical velocity limit for a copper or an aluminum slug-shaped, unloaded armature having a length of about 1 cm is several tens of kilometers per second. The theoretical limit increases proportionally to the armature length. Experimentally, the sliding contact experiences arcing at velocities about or below 1 km/s. No increase of velocity with armature length is found.
In recent years, understanding in the causes of arcing at the sliding contact has revealed two principal causes. First, strong repelling forces between the rails cause the rails to deflect, creating gaps between the sliding armature and the rails. Second, velocity skin effect results in concentration of current near the trailing edge of the contact zone causing thermal degradation of the armature.
The repelling forces are applied mainly behind the armature, but they generate elastic deformations traveling along the rails which may outrun the armature and deflect the rails. To avoid the gaps and to maintain a tight contact, the rail deflections should be minimized. Prior teachings suggest increasing the stiffness of the railgun barrel structure to withstand the repelling forces. Techniques have been developed to provide for sufficient structural stiffness of barrels, resulting in a high barrel mass per unit length.
Even with stiff barrels, metal armatures should be designed to maintain the electromechanical contact during the launch. Armatures are usually inserted in the bore with substantial interference. Elastic, magnetic or inertial forces are often used to maintain pressure at the contact. Although a variety of armature designs have been developed and tested, maintaining a reliable contact, especially at the elongated contact zone for launching massive payloads to high velocities, remains a problem.
Moreover, if the sliding contact pressure occasionally becomes too high, gouging may occur. Excessive contact pressure may occur when the projectile begins to ballot within the bore. Further, high current density at the sliding contact causes superheating which degrades the mechanical properties of the armature and rail materials near the contact, which facilitates gouging. Gouging may greatly damage the contact surfaces of the rails and the armature and impede achieving high velocities.
The second cause of degradation of the sliding contact is the velocity skin effect. The velocity skin effect results in current concentration near the trailing edge of the contact area. The characteristic width of the current concentration zone depends upon the resistivities of the rails and the armature. The width of the zone decreases with increasing velocity. At a velocity of 1 km/s, the width of the zone may be of order of 1 mm or less. Extremely high current density in the zone causes thermal degradation/melting of the armature material in the vicinity of trailing edge of the sliding contact. This results in the destruction of the initially tight contact near the trailing edge.
As the armature material degrades, causing a gap between the armature and the rails, the zone of current concentration shifts to the trailing edge of the undamaged contact area. The entire process continues, causing a wave of contact degradation to propagate from the trailing edge of the contact zone toward the leading edge. When the entire contact is destroyed, arcing transition occurs. Theoretical models of this process were presented in P. Parks, Current/Melt Wave Model for Transitioning Solid Armature, J. Appl. Phys., Vol. 67, No. 7, pp. 3511-3516 (April 1990); T. James, Performance Criteria for EM Rail Launchers with Solid or Transitional Armatures and Laminated Rails, IEEE Trans. Mag., Vol. 27, No. 1, pp. 482-489 (January 1991); and Y. Dreizin, Solid Armature Performance with Resistive Rails, IEEE Trans. Mag., Vol. 29, No. 1, pp. 798-803 (January 1993). The velocity skin effect was also studied by a number of other authors, the first systematic analysis being published by Young and Hughes, Railgun and Armature Current Distributions in Electromagnetic Launchers, IEEE Trans. Mag., Vol. 20, No. 2, pp. 33-41 (January 1982).
A number of methods were proposed to diminish current concentration due to the velocity skin effect. One proposal concerns increasing the armature resistivity and using laminated (chevron type) armatures to reduce the current concentration. However, the zone of thermal degradation of the armature material near the contact becomes deeper in more resistive armatures. This increases the risk of destroying the armature during the launch. R. A. Marshall proposed "chevron" rails in, The use of Nested Chevron Rails in a Distributed Energy Store Railgun, IEEE Trans. Mag., Vol. 20, No. 2, pp. 389-390 (March 1984). The chevron rails use a number of independent power sources with short current pulses to control plasma armature behavior.
Another approach to solve the problem of arcing at the sliding contact is based on an augmented railgun. The augmented railgun involves a trade-off between the current passing through the armature and the magnetic field strength. If the current is decreased by a certain factor and the magnetic field is increased by the same factor, then the electromagnetic driving force will remain the same and the thermal effect will become smaller. However, limitations in the magnetic field strength due to the finite strength of the materials of the railgun structure do not completely allow for the trade-off just described. Instead, the magnetic field usually remains approximately the same (near the maximum possible level) while the current is reduced and the railgun is made longer to compensate for the reduction of driving force. To provide for a high magnetic field with the reduced current, additional rails are used, parallel to the main rails and energized from the same power source or from separate sources.
A self-augmented railgun is described in R. Burton, F. Witherspoon & S. Goldstein, Performance of a Self-Augmented Railgun, J. Appl. Phys., Vol. 10, No. 7, pp. 3907-3911 (October 1991) and in J. Parker, Muzzle Shunt Augmentation of Conventional Railguns, IEEE Trans. Mag., Vol. 27, No. 1, pp. 80-84 (January 1991). In a self-augmented railgun, a part of the current supplied by the power source at the breech end is not closed by the armature circuit, but is transported to the muzzle end where the circuit is closed by a conducting shunt. The magnetic field in front of the armature can be commensurate with the magnetic field behind the armature, so the average magnetic field at the armature increases as the armature moves from the breech end to the muzzle end. This partly compensates for the current reduction in the armature. For example, if the current is divided equally between the armature and the muzzle shunt, then the driving force is decreased only by one fourth while the thermal effects in the bulk of the armature and at the contact become four times lower.
Several researchers have experimented with multi-armature, multi-rail railguns. In such railguns, two or more pairs of parallel rails extend along the bore and may be energized independently. A corresponding number of armatures are disposed along the projectile. Each armature contacts one pair of rails. This configuration helps to distribute the electromagnetic driving force over the length of the launch package, but the arcing transition problem remains essentially the same.
Another method for reducing current concentration at the armature-rail contact was proposed in U.S. Pat. No. 4,953,441. There, the railgun comprises a laminated rail consisting of a high conductivity layer (copper) and an adjacent low conductivity layer. The low conductivity layer has a sliding contact with the armature. The high conductivity layer transports the current from the breech end of the rail to the location of the armature, while the low conductivity layer controls the current distribution at the contact to prevent current concentration. Certain difficulties, including superheating and thermal degradation of the low conductivity layer due to Joule dissipation, would be anticipated with the use of laminated rails.
Another problem in electromagnetic launch technology relates to the pulsed power supply. The performance of conventional electric sources and inductive storage devices is limited by the specific strength of available construction materials. It is difficult to expect cardinal progress in the specific strength or in the weight of those sources and devices.
A number of articles describe alternative methods of energizing electromagnetic launchers in general and railguns in particular. In recent years, significant progress was made in capacitor storage technology. This is undoubtedly the most convenient method to store electric energy. However, even the most bold forecasts for the specific energy of capacitor banks are still lower than that of inductive energy storage.
In the early 1980's, research programs studied electromagnetic launchers at the Livermore and Los Alamos National Laboratories. The programs focused on railguns energized by magnetic flux compression generators (MFCG). MFCGs are inexpensive one shot devices used as pulsed power sources to energize various loads, usually in the submillisecond range of pulse duration. The simplest strip type MFCG consists of two elongated, generally parallel strips of high conductive metal directly connected to the load. The strips may be initially shorted at one end by a switch and connected at the other end to an external pulsed electric source (a capacitor bank or another MFCG). After the external source generates a short current pulse and thus supplies the strip circuit with magnetic flux, an explosive positioned adjacent one of the strips is initiated near the power source end of the strip. A detonation wave propagates along the strip, imploding one strip onto the other. The implosion creates a traveling crowbar effect which diminishes the length and inductance of the strip line, thus compressing the magnetic flux and increasing the electric current and magnetic energy. The current in a MFCG can easily reach several megaamps.
MFCGs can be connected in series, forming a cascade, in which one MFCG section feeds the next section with magnetic flux and energy. Transformer links between the sections have been used in such cascades to increase the magnetic flux. MFCGs have been successfully used to launch projectiles in small and medium caliber railguns. Because of the short current pulse produced by the MFCG, the railgun barrels were relatively short, and projectiles were often damaged because of the high acceleration they were subjected to.
Railguns and strip line MFCGs geometrically are similar. D. R. Peterson and C. M. Fowler published a short description of the conceptual design of a hybrid railgun called an "integral railgun". D. Peterson & C. Fowler, Rail Gun Powered by an Integral Explosive Generator, Los Alamos Nat. Lab. Rep. LA-8000-C; See also, C. Fowler et al. Explosive Flux Compression Generators for Rail Gun Power Sources, IEEE Trans. Mag., Vol. 18, No. 1, pp. 64-67 (January 1981). According to the authors, metal strips or elongated plates are used as guiding and contacting rails of a conventional railgun and as conducting and imploding strips of the MFCG. Fowler and Peterson studied a hybrid railgun operation using a simplified analytical model, in which flux losses were neglected. They have shown that, ideally, such a system can be used to launch a projectile to hypervelocities.
However, Fowler and Peterson recognized potential problems inherent to the conceptual design they considered. In addition to flux losses, the design may generate harmful jets. Jets occur when the rails undergo high velocity collision at a small angle (a process somewhat similar to shaped charge implosion). The generation of hypervelocity jets in such collisions is dangerous because the jets can easily reach and destroy the projectile.
One additional potential problem not discussed by Fowler and Peterson is maintaining a gapless contact with a metal armature, or a gapless sealed bore with a plasma armature. Large magnetic pressure in the region between the armature and the rail crowbar will expand and deform the strips, creating gaps in the contact.
The above discussion shows that despite certain very enticing advantages of a hybrid railgun, the conceptual design has not been readily implemented in an effective and practical launcher. While it is desirable to integrate electric energy generation and projectile acceleration in one device, a feasible conceptual design should eliminate the above mentioned problems.